Electrically heated vacuum furnace



A ril 7, 1964 K. J. HINTENBERGER 3,128,326

v ELECTRICAL-LY HEATED VACUUM FURNACE Filed Feb. 28, 1961 3 Sheets-Sheetl H u INVENTOR lllll ATTORNEY April 7, 1964 Y K. J. HINTENBERGER3,123,326

ELECTRICALLY HEATED VACUUM FURNACE Filed Feb. 28, 1961 3 Sheets-Sheet 2I N VENTOR Km; Jam flnvmvamsre ATTORNEY United States Patent r 3,128,326ELETRICALLY HEATED VACUUM FURNACE Karl Josef Hintenherger, Hochstrasse39, Frankfurt am Main, Germany Filed Feb. 28, 1961, Ser. No. 92,370Claims. (Cl. 13--31) This invention relates to an improved electricallyheated vacuum furnace used particularly in denture ceramics, torinstance, to bake, fire and glaze artificial teeth, crowns, and thelike, and generally, where the items to be processed in the furnace aremade of ceramics and are fused, or dried, at reduced atmosphericpressure in a chamber accommodating the heater mufiie.

More specifically, the invention relates to assemblies comprising aninstrument case and a furnace mounted thereon, where the instrument casecontains a vacuum pump and driving means for the same as well asswitching and control devices for all power-consuming units while thefurnace comprises a vacuum chamber mounted in spaced relationship to andabove the chest-like instrument case, the furnace chamber beingconnected by a conduit or pipe to a vacuum pump. The furnace comprises avertically extending inverted cup-shaped chamber heat-insulated orshielded at the sides and the top, but open in the lower face which isnearest the instrument case. A charge elevator also is supported by theinstrument case for vertical movement toward and from the top so thatvthe furnace charge placed on a horizontal table of the elevator ismoved from a loading position into the furnace, the table also havingmeans to seal the furnace aperture.

7 Vacuum furnaces of the type above described have several advantagesover those where the charge is inserted into the furnace from the top orfrom one side, these being, (1) the loss caused by heat radiation isreduced due to the fact that the furnace top has no aperture at its topside, and (2) the operator is not discomforted by the heat radiated fromthe furnace charging opening.

Another advantage of the type of vacuum furnace comprising the inventionis that the charges placed within the furnace chamber always moveidentical short distances and in a uniform, smooth manner, resulting inuniform products, the homogeneity being enhanced as a result of theelimination of the conventional necessity of removing and inserting, bytweezers, shielding bricks and heat insulators into and from thefurnace, respectively.

Itis an object of this invention to provide an improved vacuum furnacein which the operations are automatically controlled and especially themanual work is simplified over that required in conventional porcelainfiring furnaces so as to eliminate human errors and irregularitiesresulting from manual handling, as well as [faulty operationparticularly under conditions where a substantial series of successivefirings are required.

Another object of this invention is to eliminate some of the controlfunctions previously required to be performed by human operators, thussimplifying the mode of operation, reducing manual work to a minimum andproviding greater convenience and reliability.

Still another object of this invention is to provide electrical circuitsand electrical devices adapted to provide proper timing and control ofindividual operations.

A further object of this invention is to provide cooperative functioningof the vacuum and pressure ends of the vacuum pump by means ofelectrical and pneumatic mechanism so as to conveniently andautomatically control and perform mechanical operations.

Another object of this invention is to provide a vacuum furnace withadjustable means to control the various mechanical, electrical andpneumatic means which may 3,128,326 Patented Apr. '7, 1 964 ICC beregulated and interrelated without interference or impedance.

Details of the foregoing objects, and others, are set forth in thefollowing specification and are illustrated in the accompanying drawingsforming a part thereof.

In the drawings:

FiG. l is a front elevation of a vacuum furnace according to the presentinvention and showing the elevating apparatus in its lowered position,prior to being elevated.

FIG. 2. is a schematic side elevation of a control system according tothe invention for moving the elevating apparatus up and down by means ofthe vacuum and pressure produced by the vacuum pump of the device.

FIG. 3 is a circuit diagram of the instruments and controls of thepreferred embodiment of the invention.

FIG. 4 is a vertical sectional view, partially in perspective, of therfiurnace and especially showing details of the insulated firingchamber, including the columns upon which the furnace is supported anddetails of the connection between the furnace and the instrument casing,and

FIG. 5 is a fragmentary vertical elevation, partially sectioned, showingdetails of the detachable connection between the columns carrying thefurnace and the instrument casing, on a larger scale than in FIG. 4.

Referring to FIG. 1-, there is shown a supporting base comprising ahousing or case 1 containing the instruments, and a furnace 2. Thefurnace 2 has an internal firing or baking chamber, which is an invertedcup, supported on columns 3 a predetermined distance above case 1. Thefront side of case 1 contains the dials of the various switching andcontrol units, such as the operating handles and dials of two timingswitches 4 and 5, a voltmeter 6, a transformer 7, a toggle type mainswitch 8, which is manually operated, bywhich the entire circuit iscontrolled, and two pilot lamps 9 and '16]. The furnace is provided witha vacuum gage 11. Hose 12' is connected, at one end, to a vacuum pump20, see FIG. 2, arranged inside of the instrument casing, and, at itsother end, to a control valve, not shown, mounted at the rear of thefurnace Z.

The elevating apparatus 13 is supported by casing 1 and consists of apneumatic cylinder 26 and piston 27:, and table 14 connected to theupper end of piston 27, said table also serving to hermetically closethe opening in the bottom of the furnace chamber. For this purpose, aseal 15 of silicone rubber is arranged on the, perimeter of table 14;.Concentrically arranged on the table 14 within seal 15 is an asbestoswasher 16 which abuts against the rim of the opening in the furnace. Acylindrical charge carrier base 17, which carries three columnssupporting an upper disc portion, is mounted coaxially on the table 14.This charge carrier base 17 is formed of ceramic material such as fireclay. A charge carrier 18' is provided which may consist of a bowl andsupporting members, if desired. A charge to be fired is placed with inthe carrier 18 and may, for example, consist of a ceramic artificialtooth or crown. I

To perform the firing process according to the present invention, theelevating apparatus 13 is operated to elevate table 14 by means ofpneumatic piston 27, after the charge has been inserted into the carrier18. Although this movement may be manually performed by a suitablehandle rod 19, the invention contemplates prefer-ably that the piston 27be moved automatically by means of air' pressure from the pressure sideof the vacuum pump 20. Cylinder 26, as shown in FIG. 2, communicates atits bottom with a pipe in which a ball check valve 34 is mounted belowthe bottom of the cylinder, and a vent hole 35 is formed in saidpipebetween the cylinder and the ball check valve, said vent hole having adiameter substantially smaller than that of the pipe. Said pipe is.

connected by means of another pipe 25 to the pressure side of the vacuumpump it and has a valve block 28 connected therein which is providedwith an opening having a valve seat 29. Similarly, the suction pipe 24connected to the suction side of the vacuum pump 26 leading to thefurnace has a valve block 24a having an opening also provided with avalve seat. B-oth valve blocks 24a and 29 are closed or opened byelectrical means provided for that purpose, to be described.

Referring now to FIG. 2, the vacuum; pump 23 is shown having a suctionpipe 21 provided with a hose, connector 22 to which the hose 12 shown inFIG. 1 is connected. Furthermore, a connecting nipple 23 is provided atthe pressure side of the pump. Air pressure line 25 is connected to theconnector 23 at the pressure side of the pump 20 and leads to the hollowcylinder 26 in which piston 27 is closely slidable for moving thelifting apparatus. A double headed valve member 31 is movablereciprocably in a guide member 30 between valve seat 24a which controlsflow to suction line 24 and valve seat 29 which controls flow topressure line 25. Movement of valve member 31 into seat 24a is caused bymeans of a retracting spring 32. Upon energization of an electricalrelay 33, described hereinbelow, the double-seat valve member 31 ismoved away from valve seat 24 a, against the action of the retractingspring 32, and is pressed against the valve seat 29 of the valve block28. p The arrangement which has been described above is used to producethe following results relative to the firing furnace comprising theinvention. When the vacuum pump of the system is started, preferablyafter the furnace 2 has been heated to firing temperature, the doublehead valve 31 is moved against the valve seat 2? of the valve block 28through energizati-on of relay 33, whereby the vacuum pipe 24 is openedto atmosphere, and air under pressure from nipple 23 is transmittedthrough line 25, through ball check valve 34, thereby elevating the ballof said valve and acting on the piston 27 to elevate it and the table 14toward the entrance opening in the lower end of the furnace 2. In thismanner it is possible, smoothly, and without jamming, to move the chargecarrier together with the charge into the baking chamber, in a highlyimproved manner over previously used manual insertion.

Upon de-energization of the relay 33, spring 32. retracts the doubleseat valve to its normal seat position against seat 24a, whereby thevent line of the vacuum pipe is closed and vacuum is produced in theinterior of the furnace 2 to draw table 14 tightly against the furnaceopening, since the vacuum rapidly increases to accomplish this. Althoughpressure line 25 is opened by the aforementioned shifting of valvemember 32, table 14 will not drop quickly in any event, prior to theaction of the vacuum, because of the limited air discharge through smallvent hole 35 and the action of ball check valve 34 which is in its seatunder such conditions. Relay 33 which can for example be of theelectromagnet type, may be replaced by any other suitable mechanism toreverse the double headed valve member 31. It is also possible to useother types of electrically controlled devices, such as slide valves, toalternately close and open the valves in the valve blocks 24a and 29.

Reversal of the double headed valve member 31 can be further improved bya time switch, such as a timing apparatus connected in conventionalmanner to a suitable electric switch, the time switch being connected inseries with the circuit which supplies current to the motor of thevacuum pump and to other components, if necessary, and being adapted toreverse the movement of the double headed valve member rather than byusing the above mentioned relay or other electric switch. As the circuitof the motor for the vacuum pump, to which other devices such -as thefurnace heating elements and its controls are connected, is closed byactuating a main switch 8 which is provided on the instrument casing andis connected to a suitable circuit of a source of electrical energy,through the means of an electric plug 36 as shown in FIG. 3, forexample, and the timing switch 4 has been set for a desired period ofvacuum firing time, produces a desired degree of vacuum in the furnace,the relay 33 will be energized by timing switch 4 to connect pressurepipe 25 to cylinder 26 and thereby cause the elevating apparatus to movetable 14 upwards. Such vacuum atmosphere in furnace 2 is maintaineduntil the expiration of the time as set by the timing switch 4,whereupon the circuit to the vacuum pump motor is interrupted, so thatthe vacuum pump is stopped, and the elevating apparatus gradually willbe lowered as the air from cylinder bleeds through hole 35, asdescribed.

The foregoing description refers to the time switch 4 in the circuit ofthe vacuum pump and the other aforementioned units by which the circuitthereto is closed at the instant the time switch is closed andinterrupted after a predetermined period of time. Further advantages resalt from the inclusion of a second timing switch 5 which is connectedin series with this first timing switch, the contacts of the secondtiming switch being arranged so that a circuit leading from said secondtiming switch and through the control relay 33 is opened at the instantwhen the switch starts to run, and is closed upon termination of therunning time of the switch. Relay 41 is arranged to reverse the doubleheaded valve member 31 in the manner described above, as will be seenfrom FIG. 3 of the drawings.

In FIG. 3, there is also illustrated an electric plug 36 which is on aflexible electric cord extending from the instrument casing 1 and isconnected in the circuit with main switch 8 mounted on the instrumentcasing and also with a time delay timer switch 37 conveniently mountedon case 1. The main switch 8 serves to close the main circuit 38 whichcontrols the operation of the motor of the vacuum pump 21), and themuflle heater 40 within the furnace 2 for heating the chamber thereof,and another relay 41 which functions as a temperature-sensitive switchof conventional type and is adapted to interrupt the current supply tothe furnace heater 4t) automatically upon the chamber reaching apreselected maximum temerature. To achieve such interruption of thecurrent, a pyrometer 52 is mounted in the furnace chamber and isconnected in the heater circuit. The pyrometer is connected to the relay41 by leads 43 and 44 shown in FIG. 3.

The variable transformer '7 serves to infinitely vary the heater voltageand is provided with scale divisions which are identical with those onvoltmeter 6. Excessive or under voltages are compensated for bytransformer 7. Pilot lamp 9 of a certain color serves to indicate thatthe circuit to the furnace heater 40 is ON, as controlled by the actionof the relay 41. Another pilot lamp 10 of a different color indicatesthat the circuit of the entire system is energized.

The contacts of timing switch 5 are arranged so that circuit 38 as wellas conduit 50, which is connected with in said circuit between timingswitch 4 and relay 33 (see FIG. 2), will be closed when timing switch 5starts to run. After expiration of a predetermined running time, timeswitch 5 interrupts the entire circuit 38. The contacts of the secondtime switch 4 are so arranged that conduit 50, which is connected withrelay 33, will be interrupted when the switch 4 starts to run, and willbe closed again after expiration of a predetermined running time forwhich time switch 4 is set.

This circuit arrangement permits the device to operate as follows: Whenthe furnace has been put into operation by closing switch 8 and has beensuitably heated by the heater 4%, and the charge has been placed on thecharge carrier table 14 of the lifting apparatus, time switch 5 is setfor the total desired heating time for furnace 2. Since the device, asdescribed above, provides for automatic control of the entire processcommencing from the insertion of the charge until the removal of thefinished material, and also including in the time cycle control of theoperation time and duration of the vacuum atmosphere within the furnace,followed by restoration of atmospheric pressure during the remainingfiring period, the second time switch 4 is preset for a given timeinterval to control the vacuum firing period duration.

Under the foregoing circumstances, the circuit for heating the furnacechamber 2 includes the variable transformer 7 and the motor for thevacuum pump is started directly upon closing timer switch 5. Relay 41has a rocking mercury switch which is ready to operate. By starting pump20, the valve seat 29 of valve block 28 of the pressure line ismaintained closed by the double headed valve member 31 as long as timeswitch 4 is not actuated. Piston 27 now moves upward to elevate thecharge into the furnace. Upon time switch 4 closing, pressure line 25 isopened by movement of valve member 31 away from seat 29. However, theelevating apparatus remains in its upper position, since it is held inplace by the suction action of the vacuum within the furnace chamber.Upon expiration of the predetermined running time of switch 4, thecircuit leading from this timing switch to relay 33 is again closed sothat the lifting apparatus will remain in its upper position because ofthe application of pressure to piston 27. By reversing the double headedvalve member 31, the furnace is opened to atmospheric air, whereby thefurther firing is accomplished under atmospheric pressure untilexpiration of the running time of timing switch 5, whereupon the currentsupply to the vacuum pump is interrupted. The lifting apparatus thenslowly lowers and permits the removal ofthe baked charge.

A bypassing switch 42 serves to bridge the timing switch 5 past one ofits poles when the furnace is to be heated up to a predeterminedtemperature. By this bridging arrangement, it also is possible todispense with the automatic interruption of the heating circuit. In suchcase, which is of practical importance in performing several successivefirings, the muffle remains constantly heated, it being controlled onlyby the relay 41.

The invention also permits firing under atmospheric pressure alone,without including vacuum, by using only time switch 5, whereby the pump20 and the relay 33 are energized so that the elevating apparatus raisestable 14. Upon expiration of the running time of time switch 5, both thepump and the relay are de-energized, thus causing the table 14 to movedownward.

For accomplishing the entire baking process under vacuum it will benecessary first to activate timing switch 5 and shortly afterwards,switch 4. Both switches must be set for the same period of elapsed timeunder such circumstances. However, provision should be made that timeswitch 4 is actuated a little earlier than switch 5 so that relay 33will reverse when intended and, hence, insure elevation of the table 14.This small time differential is necessary, since, if time switch 5should open earlier, the valve controlled by relay 33 would remainclosed against its seat.

FIG. 4 and the enlarged partial view of FIG. 5 illustrate how the spacebetween the instrument case 1 and the furnace can be easily adjusted toset the proper stroke length for piston 27 which is very importantrelative to automatic control of the elevating apparatus to insure thatthe table 14 may positively engage the rim of the furnace opening. FIG.4 also illustrates certain details of the design of the furnace 2. Themetal casing 53 of furnace 2 is open at its upper end and is closed by acover which is partly broken away in the illustration. Heat-insulatingbricks 56 are arranged in such manner that the heater 40 is held inplace therein within helical grooves which are disposed at a certaindistance from the bottom entrance opening of the firing chamber 57.

A tube projecting from the pyrometer extends through an opening in thebricks 59 which line the firing chamber 57. The circuit leads 43 and 44of the pyrometer 52 are connected to the relay 41 (FIG. 3), as describedhereinbefore. A cap brick 60, which is shown partly broken away, isformed of ceramic insulating material and surrounds the bricks 59through which the pyrometer extends. The casing 53 has a bottom flangewhich is secured to columns 3 upon which collars 62 are mounted. Cover54 of the casing is securely connected to an upper flange of the casingby knurled cap nuts 63 which engage the upper threaded ends of upperextensions on the columns 3 which extend through openings in the flangeand the cover 54. I

The lower ends of the columns 3 preferably are inserted into guide means64 (FIG. 4) in the instrument case 1 and are provided with threaded endportions 65 (FIG. 5) having supporting nuts 66 thereon. By adjusting thenuts, columns 3 can be adjusted vertically relative to nuts 66 and theguide means 64 in general.

The columns 3, or at least one of them, should preferably be hollow inorder to receive electric leads 69 for connection of electricallyenergized items in the furnace 2 with the controls in the instrumentcase 1. For this purpose, the columns 3 are preferably of the plugtypeso that the entire furnace can be easily removed from the instrumentcasing. These plug-type columns have moreover the advantage that, whenthe furnace is placed upon the instrument case, the electricalconnection is made automatically also. For this purpose, the lower openends of the columns 3 are provided with insulators 67 which are lockedin the columns by set screws 68. Moreover, the leads 69 extendingthrough the columns are provided with split, elastic metallic plugs 70.The cover of the instrument case also contains guide members for theinsulators, each guide member consisting of an insulating ring 71secured to the cover from below, and a bottom portion 72 of the sameinsulating material, both parts of which are secured to the cover byscrews 73. A metal bushing 74 in the bottom plate '72 serves to connectconduit 75, comprising part of circuit 38, with conduit, 69 within thecolumn.

I claim:

1. An electrically heated vacuum furnace particularly for fusing dentalceramics and comprising in combination, a base housing, an invertedcup-shaped furnace chamber supported above said base with the entrancefacing said base, a vacuum pump connected by conduit means to theinterior of said furnace chamber, the sides and the top of the furnacechamber being insulated for heat retention, a charge elevator movable inopposite vertical directions above said base housing toward and from theentrance in the bottom of the furnace chamber and provided at its upperend with a charge carrying horizontal table adapted to fit against theentrance of the furnace vessel to seal the entrance opening tightly whenthe table is lifted into engagement with the said aperture, the lowerendof said charge elevator having a piston slidably movable within avertical cylinder for movement in opposite directions, an air conduitconnected to the lower end of the said cylinder, a ball check valvemounted on said conduit, said conduit having between this valve and thesaid cylinder a bleeder hole of a diameter substantially less than thatof said conduit, the other end of said air conduit being connected tothe pressure side of the said vacuum pump, a conduit extending from saidair conduit to receive air under pressure therefrom and having a valveseat at the outer end thereof, another branch conduit extending fromsaid vacuum conduit interconnecting the vacuum pump and the said furnacevessel and having a valve seat at the free end thereof, and valve meansoperable relative to said valve seats successively first to close saidvalve seat in said first-mentioned branch conduit to cause air pressureto raise said piston and charge-carrying table against said entrance ofsaid furnace to close it and said valve means then being movable toclose said valve seat in said second mentioned branch conduit to closesaid vacuum conduit to said furnace chamber to maintain said tableagainst said furnace entrance while said air pressure is released fromsaid piston.

2. The furnace according to claim 1 further including a solenoidconnected to said valve means and operable to actuate the same to closesaid air pressure conduit, and spring means connected to said valvemeans to move it to close said vacuum conduit.

3. The furnace according to claim 1 further characterized by said valvemeans comprising a valve member reciprocable in opposite directionsbetween said valve seats, means to reciprocate said valve member in thesequence aforesaid, and timer means operable automatically to controlthe interval between such reciprocating movements of said valve memberfirst to close the furnace and next to apply vacuum to the furnace.

4. The furnace according to claim 3 further including electric heatingmeans for said furnace, circuit means to connect the same to a source ofcurrent, and a second timer switch connected in said circuit in serieswith said a timer switch for said valve member, said second timer switchbeing operable to control the period of time the furnace is heated.

5. The furnace according to claim 4 further including an electric motorto drive said pump and said motor being connected in the circuit of saidelectric heating means for said furnace and controlled by the timerthereof, whereby when said heating means is shut off by said timertherefor said pump will stop and permit said chargecarrying table todescend by the dispelling of air from said cylinder thereof through saidbleeder hole in said air pressure conduit.

References Cited in the file of this patent UNITED STATES PATENTS1,617,359 Westberg Feb. 15, 1927 1,678,875 Rohn July 31, 1928 1,994,917McGregor Mar. 19, 1935 2,966,537 Witucki et a1 Dec. 27, 1960 3,075,263Iuckniess et al Jan. 29, 1963

1. AN ELECTRICALLY HEATED VACUUM FURNACE PARTICULARLY FOR FUSING DENTALCERAMICS AND COMPRISING IN COMBINATION, A BASE HOUSING, AN INVERTEDCUP-SHAPED FURNACE CHAMBER SUPPORTED ABOVE SAID BASE WITH THE ENTRANCEFACING SAID BASE, A VACUUM PUMP CONNECTED BY CONDUIT MEANS TO THEINTERIOR OF SAID FURNACE CHAMBER, THE SIDES AND THE TOP OF THE FURNACECHAMBER BEING INSULATED FOR HEAT RETENTION, A CHARGE ELEVATOR MOVABLE INOPPOSITE VERTICAL DIRECTIONS ABOVE SAID BASE HOUSING TOWARD AND FROM THEENTRANCE IN THE BOTTOM OF THE FURNACE CHAMBER AND PROVIDED AT ITS UPPEREND WITH A CHARGE CARRYING HORIZONTAL TABLE ADAPTED TO FIT AGAINST THEENTRANCE OF THE FURNACE VESSEL TO SEAL THE ENTRANCE OPENING TIGHTLY WHENTHE TABLE IS LIFTED INTO ENGAGEMENT WITH THE SAID APERTURE, THE LOWERENDOF SAID CHARGE ELEVATOR HAVING A PISTON SLIDABLY MOVABLE WITHIN AVERTICAL CYLINDER FOR MOVEMENT IN OPPOSITE DIRECTIONS, AN AIR CONDUTICONNECTED TO THE LOWER END OF THE SAID CYLINDER, A BALL CHECK VALVEMOUNTED ON SAID CONDUIT, SAID CONDUIT HAVING BETWEEN THIS VALVE AND THESAID CYLINDER A BLEEDER HOLE OF A DIAMETER SUBSTANTIALLY LESS THAN THATOF SAID CONDUIT, THE OTHER END OF SAID AIR CONDUIT BEING CONNECTED TOTHE PRESSURE SIDE OF THE SAID VACUUM PUMP, A CONDUTI EXTENDING FROM SAIDAIR CONDUIT TO RECEIVE AIR UNDER PRESSURE THEREFROM AND HAVING A VALVESEAT AT THE OUTER END THEREOF, ANOTHE BRANCE CONDUIT EXTENDING FROM SAIDVACUUM CONDUIT INTERCONNECTING THE VACUUM PUMP AND THE SAID FURNACEVESSEL AND HAVING A VALVE SEAT AT THE FREE END THEREOF, AND VALVE MEANSOPERABLE RELATIVE TO SAID VALVE SAEATS CUSSESSIVELY FIRST TO CLOSE SAIDVALVE SEAT IN SAID FIRST-MENTIONED BRANCH CONDUIT TO CAUSE AIR PRESSURETO RAISE SAID PISTON AND CHARGE-CARRYING TABLE AGAINST SAID ENTRANCE OFSAID FURNANCE TO CLOSE IT AND SAID VALVE MEANS THEN BEING MOVABLE TOCLOSE SAID VALVE SEAT IN SAID SECOND MENTIONED BRANCH CONDUIT TO CLOSESAID VACUUM CONDUIT TO SAID FURNACE CHAMBER TO MAINTAIN SAID TABLEAGAINST SAID FURNACE ENTRANCE WHIE SAID AIR PRESSURE IS RELEASED FROMSAID PISTON.